1. Field of the Invention
The present invention relates to a power transmission device for transmitting the rotational force of a motor to a driven member. In particular, the present invention relates to a power transmission device adapted to be used in a film transport device for an optical apparatus, such as a camera or a film viewing apparatus.
2. Description of Related Art
Hitherto, an automatic-winding-type camera has a film transport device in which the rotational speed of a motor is reduced by a plurality of gears so as to rotate a film winding spool by a large rotational force.
Recently, the rotational forces of motors have been reduced as the motors and batteries have been reduced in size due to reduction in sizes of cameras. Therefore, the speed of the motors must be reduced at a large reduction ratio so as to rotate spools by a desired rotational force. In order to reduce the rotational speed of a motor, a plurality of two-stage gears, each of which is composed of two gear parts coaxially integrally formed to have different numbers of teeth, are generally used. The following methods using the two-stage gears to obtain a large reduction ratio are known.
(i) A method in which a large number of two-stage gears are used.
(ii) Another method in which a relatively small number of two-stage gears are used, each having gear parts differing greatly from each other in the number of teeth.
In power transmission through gears, power loss during transmission generally occurs at engaging parts of the gears. Therefore, as the number of engaging parts, that is, the number of gears, increases, the efficiency in power transmission decreases. Accordingly, in the method (i) described above, there is a problem in that although a large reduction ratio can be obtained, the efficiency in power transmission is lowered due to the large number of two-stage gears, the rotational force transmitted to a spool thereby becoming small. In order to overcome this problem, the reduction ratio may be further increased. However, when the reduction ratio is excessively large, even if the desired rotational force is transmittable to the spool, the rotational speed of the spool becomes very low, so that the film winding speed would be lowered.
On the other hand, in the above method (ii), if the number of gears used is reduced by using the two-stage gears having a large difference in the number of teeth between the two gear parts constituting each two-stage gear, the efficiency of power transmission can be prevented from being lowered. However, the larger gear part of the two gear parts constituting the two-stage gear must be made sufficiently large to provide such a large difference in the number of teeth, whereby the distance between axes of the two-stage gears is increased, thereby increasing the space as viewed in a direction of the axes of the gears. Then, power transmission devices described below have been proposed in order to overcome these problems.
FIGS. 3(a) and 3(b) show one of the power transmission devices as proposed. In the power transmission device shown in FIGS. 3(a) and 3(b), the rotational force of a motor 11 is transmitted by a pinion gear 12 fixed thereto to an internal gear 13a which is the larger gear part of an intermediate two-stage gear 13. Then, the rotational force of a gear 13b which is the smaller gear part of the intermediate two-stage gear 13 is transmitted to an internal gear 14a provided at a spool 14.
The power transmission device is provided with only two engaging parts of the gears, thereby significantly reducing power loss during transmission. In the power transmission device, with the internal gears used, the distance between axes of the gears does not need to be large, as compared with a case where external gears having the same number of teeth are used. As shown in FIG. 3(b), the intermediate two-stage gear 13 can be disposed close to the pinion gear 12 as viewed in a direction of the axes of the gears.
However, with the arrangement described above, although the arrangement offers an advantage regarding the winding of film by the spool 14, since the rotational force of the motor 11 in a forward or backward direction is transmitted to the spool 14 as it is, a configuration of the spool 14 must be devised, for example, to be divided into two pieces with regard to the rewinding of film.
FIGS. 4(a) and 4(b) show another proposed power transmission device. In the power transmission device shown in FIGS. 4(a) and 4(b), the rotational force of a motor 21 is transmitted by a pinion gear 22 fixed thereto to an internal gear 25a provided at a spool 25, while, during transmission, the rotational speed of the motor 21 is reduced by a two-stage sun gear 23 and, then, by a two-stage planet gear 24.
In FIG. 4(b), with a connecting arm 26 connecting the two-stage sun gear 23 and the two-stage planet gear 24 and a friction spring 27 used, the two-stage planet gear 24 revolves clockwise and counterclockwise around the two-stage sun gear 23.
When the two-stage planet gear 24 revolves clockwise and engages with the internal gear 25a of the spool 25, the winding of film can be performed, and when the two-stage planet gear 24 revolves counterclockwise and engages with a rewinding gear 28, the rewinding of film can be performed.
With this arrangement, although one gear is added to the configuration shown in FIGS. 3(a) and 3(b), the winding and rewinding of film can be performed by using a simple planetary mechanism.
With the above-described arrangement, a gear part of the two-stage planet gear 24 engages with the internal gear 25a of the spool 25 by revolving at the time of the winding of film, so that the two-stage planet gear 24 can be disposed approximately inside the internal gear 25a as viewed in a direction of the axis thereof, as shown in FIG. 4(b). That is, if an engaging gear of the spool is disposed at the outer periphery thereof, the two-stage planet gear 24 must naturally revolve in the outer area of the spool so as to engage with the external gear of the spool. Therefore, the space for the gear train must be enlarged for the revolving area for the two-stage planet gear 24. With the arrangement described above, the space can be reduced due to the provision of the internal gear 25a of the spool 25.
In the conventional power transmission devices shown in FIGS. 3(a) and 3(b) and FIGS. 4(a) and 4(b), the rotational force of the motors can be amplified and transmitted to the spools while maintaining transmission efficiency by reducing the number of gears used. However, there are the following problems regarding the xe2x80x9cbreadthxe2x80x9d as viewed in a direction of the axes of the gears, as shown in FIGS. 3(b) and 4(b), and regarding the xe2x80x9cthicknessxe2x80x9d as viewed in a direction perpendicular to the axes of the gears, as shown in FIGS. 3(a) and 4(a).
The thickness of the power transmission device shown in FIGS. 3(a) and 3(b) is the minimum required for a two-stage gear, which is the sum of thicknesses xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d required for two engaging parts, as shown in FIG. 3(a).
However, as is understandable from FIG. 3(b), the internal gear 13a of the intermediate two-stage gear 13 protrudes from the periphery of the spool 14 by an amount xe2x80x9cCxe2x80x9d, thereby increasing the breadth by the amount xe2x80x9cCxe2x80x9d.
Further, in the power transmission device shown in FIGS. 4(a) and 4(b), the two-stage planet gear 24 does not significantly protrude from the spool 25, as shown in FIG. 4(b). The power transmission device shown in FIGS. 4(a) and 4(b) offers an advantage regarding the space as compared with that shown in FIGS. 3(a) and 3(b).
However, the thickness of the power transmission device shown in FIGS. 4(a) and 4(b), which is the sum of thicknesses xe2x80x9cDxe2x80x9d, xe2x80x9cExe2x80x9d and xe2x80x9cFxe2x80x9d required for three engaging parts as shown in FIG. 4(a), is greater than that of the device shown in FIGS. 3(a) and 3(b). This is because the configuration must be devised so that a larger gear part 23a of the two-stage sun gear 23 does not interfere with the internal gear 25a of the spool 25 at a position xe2x80x9cGxe2x80x9d.
Then, in order to avoid such interference, the thickness may be reduced by reducing the size of the larger gear part 23a of the two-stage sun gear 23 so that the larger gear part 23a is lowered up to the same level in height as that of the internal gear 25a of the spool 25. However, in this arrangement, there is a disadvantage that the required reduction ratio will not be obtained.
Further, in order to avoid this problem, a larger gear part 24a of the two-stage planet gear 24 may be enlarged. However, in this arrangement, the larger gear part 24a of the two-stage planet gear 24 protrudes from the spool 25 as viewed in a direction of the axes of the gears, thereby causing the same problem as that in the power transmission device shown in FIGS. 3(a) and 3(b).
Accordingly, it is an object of the present invention to provide a power transmission device which is reduced in size in breadth and thickness and which is capable of obtaining a large reduction ratio by using a small number of gears with reduced loss in power transmission. It is another object of the present invention to provide a film transport device using the power transmission device for winding and rewinding film.
To attain the above objects, in accordance with an aspect of the present invention, there is provided a power transmission device, comprises a motor, a first gear member having an internal gear and an external gear, wherein an output of the motor is transmitted to the internal gear to rotate the first gear member, a planet gear member having a first gear part having a smaller diameter and a second gear part having a larger diameter, wherein the second gear part engages with the external gear of the first gear member, and the planet gear member revolves around an axis of rotation of the first gear member, and a driven member having an internal gear part, wherein the driven member is driven by the first gear part of the planet gear member engaging with the internal gear part of the driven member with the planet gear member having revolved.
In addition, the power transmission device further comprises a second driven member having a gear part, wherein the second driven member is driven by the second gear part of the planet gear member engaging with the gear part of the second driven member with the planet gear member having revolved in an opposite direction.
Further, in the power transmission device, the driven member is a cylindrical spool for winding a film thereon, and the motor is disposed inside the spool.
Further, in the power transmission device, the internal gear of the first gear member is larger in diameter than the external gear thereof.
In addition, the power transmission device further comprises a connecting arm for connecting the first gear member and the planet gear member.
Further, in the power transmission device, the second driven member is a film spool shaft for transporting a film provided in a film cartridge.
These and further objects and features of the present invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.